Hey Marina! I lived with a woman who was a lot like your friend. She cooked chicken plain (no sauce, no salt, no pepper, etc.), bought bulk bags of salad, drank a ton of smoothies, and even used applesauce in lieu of butter when she baked cookies. There were some serious body and food issues with this person, even though she was a stunningly lovely young woman.
One day, I made some cookies (this was my trademark recipe) and since I was running short on time, I put the batch on a plate, took a cookie, and went to class. Upon my return, our other roommate corners me and snaps, "I can't believe you didn't leave me any cookies! I've been DYING for one!"
"But... I knew you'd been waiting for them. I left the plate on the table."
We never knew for sure, but by process of elimination and past behaviors, we figured out that our super-healthy roomie must have inhaled the entire batch (we knew she didn't throw them away) when no one was around. We're talking over 30 cookies in 2 hours.
Unlike your friend, she didn't hound others about their habits, but she'd be the one constantly criticizing herself about what she's eating and the contents of EVERYTHING. So, I suspect that your friend probably has some similar issues and they're pretty deeply rooted.
With that said, my general tactic if anyone bugs me about my nutrition is to reply, "Better happy and fat than miserable and skinny. Back off." If that doesn't make them shut up, I sit there and really, really enjoy my meal. I tune them out and eat with lots of mmmm's. My indulgence is a cheeseburger with grilled onions, fries, and a neopolitan milkshake from In-N-Out. When I restrain, I have a diet coke instead of the shake. So, I think you're doing pretty well by eating your burgers with fruit! It's not the etiquette that would have me dropping your co-worker like a bad habit, it's the fact that I love to savor my meals and that's best done in peace.